1. Field of the Invention
This invention relates to a process for recovering hydrogen and nitrogen from a gas mixture containing ammonia, argon, methane and said hydrogen and nitrogen by adiabatic pressure swing adsorption.
2. Description of the Prior Art
In the synthesis of ammonia by a conventional Haber-type process, it is essential that the hydrogen- and nitrogen-containing feed gas be properly purified prior to introduction to the catalytic ammonia conversion step. Catalytic reforming of natural gas is frequently employed to provide feed gas for the ammonia conversion. Such reforming produces carbon monoxide and hydrogen from the natural gas, with the carbon monoxide being subjected to a water-gas shift reaction to yield carbon dioxide and hydrogen. Secondary reforming of the reactant gas mixture is then conducted, with compressed air being added to provide the nitrogen component of the synthesis feed gas and the oxygen in the added air being stoichiometrically reacted with hydrogen in the reactant gas mixture to form water. The addition of air is controlled in the secondary reforming operation so that the effluent gas from the reformers contains a three to one molar ratio of hydrogen to nitrogen. This effluent gas is then subjected to purification treatment as for example scrubbing by ammonia wash liquid for removal of carbon monoxide and carbon dioxide contaminants and the resulting purified synthesis gas is compressed and passed to the ammonia converter catalytic synthesis step.
In the ammonia converter, the nitrogen and hydrogen in the synthesis feed gas are reacted to form ammonia. The reaction product gas from this step is then phase separated to remove liquid ammonia product therefrom and the unreacted gas, containing substantial amounts of reactant hydrogen and nitrogen, is recycled in a circulation loop and joined with the fresh synthesis gas being passed to the ammonia converter. The gas in the circulation loop contains, in addition to the reactant hydrogen and nitrogen, small amounts of ammonia vapor, unreformed methane from the natural gas feed and argon entering with the air in the secondary reforming step. A typical composition of such circulation gas is as follows (percent by volume):
Hydrogen = 60 - 65% PA1 Nitrogen = 20 - 24% PA1 Methane = 8 - 12% PA1 Argon = 3 - 6% PA1 Ammonia = 1 - 3%
Thus, the ammonia converter circulation loop represents an accumulation point in the ammonia synthesis system for those constituents--e.g., methane and argon--which are "inert" with respect to the synthesis reaction carried out in the ammonia converter. The inert constituents accumulate in the recycle gas passed to the ammonia converter and adversely affect the overall process by lowering the yield efficiency and capacity of the ammonia converter. Accordingly, it has been common practice in the art to purge a portion of the recycle gas from the circulation loop by venting of same, so as to maintain the concentration of inert constituents at a suitably low level. For example a conventional ammonia plant based on catalytic reforming of natural gas to provide feed gas for the ammonia synthesis step may vent recycle gas from the circulation loop at a rate corresponding to approximately 6-8% of the synthesis feed gas, in order to limit the buildup of the principal inert constituents--i.e., methane and argon--to about 15% by volume. This vented purge gas is typically returned to the reformer furnaces and burned as fuel therein in order to realize the benefit of its comparatively high BTU heating value.
It is apparent from the foregoing discussion that venting of purge gas from the ammonia converter circulation loop entails a significant loss of potential reactant hydrogen and nitrogen from the loop. This loss is associated with an economic penalty for such mode of treatment inasmuch as the hydrogen and nitrogen constituents in the purge have a significantly higher value as synthesis feed gas compared to their use as fuel.
Faced with the problem of loss of the valuable reactant constituents in the vented purge gas, the prior art has in certain instances employed cryogenic separation systems for treatment of the purge gas stream to recover hydrogen and nitrogen for subsequent recirculation to the ammonia converter. Although cryogenic separation systems are able to achieve high recovery of hydrogen and nitrogen from the vented purge gas stream, the large equipment, operating and maintenance costs associated with such systems have limited their application.
Ideally, a processing system treating the ammonia plant purge gas should recover the hydrogen, nitrogen and ammonia, in that order of importance, while rejecting all of the argon and methane inert constituents. Such a processing system must operate reliably with a minimum of operating attention and have no significant adverse effect on operation of the remainder of the ammonia plant.
In an effort to provide an ammonia plant purge gas treatment system which satisfies the above-identified performance criteria and is economically attractive, the prior art has proposed the use of adiabatic pressure swing adsorption systems for the removal and recovery of the hydrogen constituent in the purge gas. Adiabatic pressure swing adsorption systems are well known in the gas separation art and have demonstrated utility in a variety of applications, e.g., the treatment of raw natural gas to remove water and heavy hydrocarbons therefrom. Unfortunately, the prior art systems developed to date for ammonia plant purge gas treatments have not been able to provide substantial recovery of both nitrogen and hydrogen constituents. Faced with such inability to recover both constituents in quantity, the prior art pressure swing adsorption systems have been designed and operated to provide high recovery of hydrogen at high purity, as for example 99.5 volume percent hydrogen. In these systems, the other synthesis feed gas constituents, i.e., nitrogen, methane, argon and ammonia are selectively adsorbed from the purge gas at higher pressure and desorbed from the adsorbent at lower pressure with the desorbate being vented from the system as waste gas to the atmosphere. The high purity hydrogen product recovered by the adsorption is then collected and recycled to the ammonia converter along with the synthesis feed gas. Such recovery and recirculation to the synthesis converter of the more valuable hydrogen constituent provides some improvement in the production capacity of the ammonia plant, but such improvement is not as great as that which might be expected based solely on a consideration of the purge gas treatment system. This is due to the fact that additional air must be added to the ammonia plant secondary reforming step in order to maintain a stoichiometric balance of nitrogen to the hydrogen in the feed gas passed to the ammonia converter. The additional introduction of air into the ammonia production process, may be disadvantageous due to existing air compressor limitations. In addition, the level of inert constituents, especially argon in the synthesis feed gas and circulation loop gas is correspondingly increased. As mentioned, increased levels of inert constituents in the feed and circulation loop gas are detrimental to the ammonia process since they reduce the capacity and efficiency of the ammonia converter. In summary, the prior art has not been able to economically remove both nitrogen and hydrogen constituents from the ammonia plant purge gas at desirable high recovery levels.
Accordingly, it is an object of the present invention to provide an improved process for separation of hydrogen and nitrogen from ammonia plant purge gas.
It is another object of the invention to provide an adiabatic pressure swing adsorption system for separation of hydrogen and nitrogen from ammonia plant purge gas at higher nitrogen recovery levels than have been achieved by the pressure swing adsorption systems heretofore employed for such purpose.
These and other objects of the invention will be apparent from the ensuing disclosure and appended claims.